The present invention relates to phosphate-bonded magnesia cements, and, more particularly, to a process and reactant for achieving uniform setting times of such cements.
Cements are hard, rock-like materials formed by reacting two or more components together, and then allowing the mixture to harden as the reaction proceeds. In the best known and most widely used type of cement, termed Portland cement, the hardening is generally based upon the reaction of calcium oxide and silicates. Such cements, which are commonly used for highway and sidewalk construction, have a "skinning time", the time before visible hardening or formation of a surface skin takes place, of on the order of an hour, and a "setting time", the time to achieving hardening throughout a volume, of hours to days. The relatively long skinning and setting times allow the preparation of large batches and the placing of the cement into forms conveniently and without undue haste, a requirement for many applications such as highway construction. Cements may also be conveniently utilized in mixtures with fillers such as fibers, rocks or sand, the product then being known as a "concrete."
Cements based on other chemical reactions, such as, for example, the reaction of magnesium oxide (also termed magnesia) with various salts, have different properties from those based upon the calcium oxidesilicate reaction. For example, the reaction between magnesium oxide and magnesium chloride produces a cement having a setting time of typically about 2 to about 8 hours. Such cements, termed Sorel cements, are often used for cement flooring in buildings.
In another type of magnesia cement, termed a phosphate-bonded magnesia cement, a phosphate such as ammonium phosphate is mixed with the magnesia, to form the cement. Phosphate-bonded magnesia cements are noteable for their very short skinning and setting times, which may be as little as two minutes and five minutes, respectively. These very short skinning and setting times allow phosphate-bonded magnesia cements to be used in many applications, such as coatings for walls, that would not be possible with cements having longer skinning and setting times, because such cements tend to move from their applied position before hardening.
In one approach to preparing phosphate-bonded magesia cements, the ammonium phosphate is provided in a liquid solution which may be conveniently prepared utilizing the technologies well known in the art of fertilizer preparation. Fertilizers which contain primarily nitrogen and phosphorus present these elements in the form of ammonium phosphate, and such fertilizers may also be used as the source of ammonimum phosphate in preparing phosphate-bonded magnesia cement. In one approach, a 10-34-0 composition having the ammonium phosphate fully dissolved into solution, and also having a large proportion of the phosphate present as polyphosphates, is widely available commercially as a fertilizer material and is also widely used commercially in the production of phosphate-bonded magnesia cements. The liquid fertilizer is mixed with the solid magnesium oxide, any fillers which are desired and possibly other ingredients, and immediately applied to the form or surface within the skinning time limitation. Setting occurs within a few minutes after application. (The "N-P-K" shorthand notation for fertilizer compositions will be utilized herein, the N denoting the percent by weight of ammoniacal nitrogen, the P denoting the percent by weight of phosophorus expressed as P.sub.2 O.sub.5, and the K denoting the potassium content in weight percent.)
The very short skinning and setting times of phosphate-bonded magnesia cements are necessary in some applications, but in other applications it is desirable to have slightly longer skinning and setting times. Several approaches have been discovered for lengthening the skinning and setting times by a few minutes, while retaining the strength properties of the hardened cement, which ordinarily decrease with increasing setting time. For example, the addition of oxy-boron or sodium chloride compounds and specification of particular phosphate chemistries in the ammonium phosphate solution have been proposed as approaches for lengthening the skinning and setting times. On the whole, such techniques do achieve some lengthening of the skinning and setting times.
It is also observed that there may be a large degree of variation in the skinning and setting times of phosphate-bonded magnesia cements, particularly when using the ammonium phosphate fertilizer materials that are mass produced and available on a widespread basis. That is, 10-34-0 compositions are prepared by many different manufactures, and the processes used to prepare the compositions may vary from manufacturer to manufacturer. Because the primary purpose of the production of such 10-34-0 solutions is for agricultural fertilizers, the manufacturers of such compositions do not standardize their manufacturing operations so as to meet the requirements of the producers of magnesia cements.
Consequently, the skinning and setting times of phosphate-bonded magnesia cements prepared using various 10-34-0 ammonium phosphate solutions available from different manufacturers, and even produced by the same manufacturer at different times, exhibit wide variations. For example, the setting times may vary from about 7 minutes to greater than 10 hours, when using nominal 10-34-0 compositions of the ammonium phosphate solution. This variability poses a significant drawback to the use of phosphate-bonded magnesia cements, because the user cannot readily predict when the cement structure will be available for use after the cement is applied. The ordinarily short setting times of the phosphate-bonded magnesia cements should allow the structures to be used shortly after the cement is applied, but this advantage is significantly impaired when variability increases the possible setting time to several hours, in an unpredictable fashion.
Accordingly, there exists a need for some approach to achieving uniform skinning and setting times for phosphate-bonded magnesia cement. To date, no such approach based in the control of chemistry has been proposed. It is conceivable that, if the causes of variation could first be discovered, extremely tight specifications could be drawn on the ammonium phosphate material, so that solutions could be prepared meeting these tight specifications and also so that incoming material could be screened to ensure that it meets the specifications. However, this approach would increase the cost of phosphate-bonded magnesia cements, by requiring that manufacturers produce a special cement-grade ammonium phosphate material, which would almost certainly result in increased price and reduced number of sources for the material. Thus, it would be desirable to have a technique for controlling the chemistry of the cement reaction to achieve relatively uniform skinning and setting times for phosphate-bonded magnesia cements, without imposing unduly narrow restrictions on the ammonium phosphate reactant. The present invention fulfills this need, and further provides related advantages.